1. Field of the Invention
The invention relates to a laser power control apparatus and a laser power control method.
2. Description of the Related Art
In recent years, in the fields of an auxiliary storage device for a computer, a consumer video cassette recorder and the like, a demand for a rewritable optical disk device has been on the rise. Generally, a semiconductor laser light source has been employed for formation of a recording mark on an optical disk. There is a necessity for a semiconductor laser to emit a light pulse for formation of a good recording mark. In order to form a recording mark, for example, a semiconductor laser is, as shown in FIG. 6, intensity-modulated between a peak power and a bottom power to perform multipulse light emission. For formation of a recording space, a semiconductor laser emits light continuously for a predetermined time at a bias power. In order to secure a stable recording performance, it is necessary to control the laser powers precisely. A power characteristic of a semiconductor laser, however, is greatly affected by an ambient temperature or the like. Accordingly, even if powers are set once prior to recording to supply given driving currents, the powers have no chance to be kept at the given values, resulting in fluctuations in the powers accompanying a rise in temperature of a semiconductor laser body or its peripheral equipment.
As a means to avoid fluctuations in the powers, it is useful to calibrate the powers in regular intervals. In an optical disk format having a sector structure, for example, there is provided a laser control region in a sector for calibrating a laser power one time at each sector. In this case, each time a light spot passes over a laser control region, a power is calibrated in the laser control region, thereby enabling fluctuations in the powers to be avoided.
A description will be given of a power calibration method in a laser control region in the conventional art using FIG. 9 below.
(a) When a light spot arrives at a laser control region, a given current lop1 is supplied to a laser. A laser power P1 at this time is detected as a laser power detection voltage Vm1 by a laser power detecting means including a light receiving element and a current-voltage conversion circuit.
(b) Then, a given current lop2 is supplied to the laser. A laser power P2 at this time is detected as a detection voltage Vm2 by the laser power detecting means. The laser power detection voltages Vm1 and Vm2 are converted by an arithmetic circuit to the respective laser powers P1 and P2.
With such a procedure applied, as shown in FIG. 10, it is possible to obtain a relationship of a laser power with a laser driving current in calibration (hereinafter the relationship is referred to “a laser I-L characteristic”). Therefore, by driving a laser with a driving current corresponding to a desired power in the recording region, the laser can be driven with a desired power (this scheme is hereinafter referred to as “continuous (DC) light emission scheme).
A problem remains in the continuous light emission scheme, however. In a case where a frequency characteristic of a light receiving element is low, a laser has to be driven for a long time at given values in order to settle laser power detection voltages Vm1 and Vm2. In a case where a laser is continuously emitted at the power P2 of the order of a recording peak power as compared with the power P1 of the order of an erase power, a recording film on an optical disk is illuminated with a continuous high power, leading to degradation of the recording film formed by vapor deposition. In each data rewriting, illumination with the continuous high power to the recording film was performed and repeated for a long time, which would result in inconveniences that not only is the laser control region degraded, but the degradation of a recording film is also spread into even a data recording region. Since light emission is repeated in each of laser control regions continuously at a high power for a long time, a lifetime of a semiconductor laser has a high possibility to be shortened.
As a method to avoid the problem as described above, there has been disclosed a method as taught in, for example, Japanese Laid-open Patent Publication No. 2000-244054. A description will be given of a power calibration method in a laser control region in Japanese Laid-open Patent Publication No. 2000-244054 with reference to FIGS. 11 and 12.
FIG. 11 is a block diagram of a configuration of a laser control apparatus in Japanese Laid-open Patent Publication No. 2000-244054. FIG. 12 is an operation sequence diagram of the apparatus.
(a) When a light spot arrives at a laser control region, a driving current lop1 is supplied to a laser by a laser driving circuit 140 to perform a continuous (DC) light emission at the bias power P1.
(b) A light emission power at this time is detected by a laser power detecting means 100. The laser power detecting means 100 is constituted of a light receiving element 101, a current-voltage conversion circuit 102, a peak detecting circuit 116, a bottom detecting circuit 115 and a multiplexer 117.
(c) The multiplexer 117 is configured so as to select a peak detection voltage y outputted from a peak detecting circuit 116, a bottom detection voltage z outputted from a bottom detecting circuit 115 and a through output x of the current-voltage conversion circuit 102 according to control signals v and w from the outside.
(d) In continuous (DC) light emission at an erase power P1, the through output x is selected by the multiplex 117 to output Vm1 as a laser power detection voltage.
(e) In succession, driving currents lop2 and lop3 are supplied while being switched therebetween to the laser to thereby perform pulse light emission between a recording peal power P2 and a bottom power P3.
(f) A light emission power at this time is detected by the laser power detecting means 100. In this situation, there is outputted a switching waveform between Vm2 and Vm3 as a through output x from the current-voltage conversion circuit 102. From the peak detecting circuit 116, there is outputted a peak detection output y in which the peak level Vm2 of the through output x is held for a given period. From the bottom detecting circuit 115, there is outputted a bottom detection output z in which the bottom level Vm3 of the through output x is held for a given period. The peak detection output y and the bottom detection output z are selected by the multiplexer 117 in a proper manner to output Vm2 or Vm3 as a laser power detection voltage.
The laser power detection voltages Vm1, Vm2 and Vm3 produced in such ways are converted to digital values by the AD conversion circuit 121 to further convert the digital values to laser powers P1, P2 and P3 with an arithmetic processor 125. Thus, an I-L characteristic of the laser can be produced by the arithmetic processor 125.
With such a construction adopted, the same output as a detection output obtainable by continuously driving a semiconductor laser itself for a long time can be produced from the peak detecting circuit 116 since the peak level Vm2 is held for a given period. In this scheme, since the semiconductor laser emits light pulses, so a laser power illuminated to a recording film is reduced as compared with a continuous (DC) light emission scheme, damage caused on a recording film decreases, thereby enabling reduction in lifetime of a semiconductor laser to be alleviated.
Even in the above scheme having the construction, there have been remained problems. In a case where a higher speed recording is tried, or in a case where a higher precision DC characteristic is requested by a laser power detecting unit and a frequency characteristic of a current-voltage conversion circuit can not be sufficiently secured, an output of the current-voltage conversion circuit does not reach Vm2 or Vm3 to thereby reduce a power detecting precision. Furthermore, in a case where a width of a light pulse is increased for the purpose to cause an output of the current-voltage conversion circuit to reach Vm2 and Vm3, there occur inconveniences such as damage caused on a recording film, reduction in lifetime of a semiconductor laser and the like in a similar way to that in the continuous (DC) light emission scheme.